An electronic device, such as a computer peripheral, may be powered from a single fixed voltage power supply and use a switch regulator to supply another voltage for driving a load circuit. A step-down or buck mode switch regulator converts an applied input voltage to a lower output voltage. A step-up or boost mode switch regulator converts an applied input voltage to a higher output voltage.
Typically, a switch regulator includes an inductor to smooth out a current passing through it. To ensure that the switch regulator is functioning properly, the current going through the inductor should be monitored and protected against some limitations. One solution for detecting the inductor current is to provide a sense resistor connected in series with the inductor and to measure the voltage drop over this resistor. Specifically, FIG. 1 illustrates a simplified schematic of a switch regulator having a sense resistor coupled in series with an inductor.
As is shown in FIG. 1, sense resistor (R) 100 is coupled in series with inductor 110 via input node 120a. Input node 120a is coupled with a positive input of comparator 130. Comparator 130 also includes a negative input that is coupled with the sense resistor 100 via output node 120b. Comparator 130 then measures the voltage drop over sense resistor 100 in an effort to measure the current going through inductor 110.
FIG. 2 illustrates another embodiment for measuring the current going through an inductor using a sense resistor. In the embodiment of FIG. 2, sense resistor 200 is coupled in series with NMOS switching transistor 240 via input node 220a. Input node 220a is coupled with a positive input of comparator 230. Comparator 230 also includes a negative input that is coupled with the sense resistor 200 via output node 220b. Comparator 230 then measures the voltage drop over sense resistor 200 in an effort to measure the current going through inductor 210. It should be appreciated by those skilled in the art that in this embodiment, the current going through inductor 210 is determined at a time when clock 250 turns on NMOS switching transistor 240 and turns off PMOS switching transistor 260.
Regardless of whether a sense resistor of a switch regulator is coupled directly with an inductor (that it is sensing) or indirectly via a transistor (or a switch), the sense resistor will reduce an efficiency of the switch regulator because the sense resistor is coupled in series with the inductor (which results in loss of power). Also, the reference setup for a comparator coupled with the sense resistor is quite complicated and troublesome to design. Moreover, since the sense resistor is typically formed using a metal piece, it can be an expensive device that has a large system footprint. Accordingly, there is a need to provide an over-current detector in a switch regulator that overcomes the above-described shortcomings of the sense resistor while retaining its advantages.